On 2015-06-02 15:15:39 +0200, Fabien COELHO wrote:
> >Won't this lead to more-unsorted writes (random I/O) as the
> >FlushBuffer requests (by checkpointer or bgwriter) are not sorted as
> >per files or order of blocks on disk?
>
> Yep, probably. Under "moderate load" this is not an issue. The io-scheduler
> and other hd firmware will probably reorder writes anyway.
They pretty much can't if you flush things frequently. That's why I
think this won't be acceptable without the sorting in the checkpointer.
> Also, if several
> data are updated together, probably they are likely to be already neighbours
> in memory as well as on disk.
No, that's not how it'll happen outside of simplistic cases where you
start with an empty shared_buffers. Shared buffers are maintained by a
simplified LRU, so how often individual blocks are touched will define
the buffer replacement.
> >I remember sometime back there was some discusion regarding
> >sorting writes during checkpoint, one idea could be try to
> >check this idea along with that patch. I just saw that Andres has
> >also given same suggestion which indicates that it is important
> >to see both the things together.
>
> I would rather separate them, unless this is a blocker.
I think it is a blocker.
> This version seems
> already quite effective and very light. ISTM that adding a sort phase would
> mean reworking significantly how the checkpointer processes pages.
Meh. The patch for that wasn't that big.
The problem with doing this separately is that without the sorting this
will be slower for throughput in a good number of cases. So we'll have
yet another GUC that's very hard to tune.
Greetings,
Andres Freund